Kb Calculator Formula

Understand the math behind the kb calculator. Each variable explained with a worked example.

Use the Kb Calculator

Formulas Used

Kb

kb = (conc_bh * conc_oh) / conc_b

pKb

pkb = -log10((conc_bh * conc_oh) / conc_b)

Variables

VariableDescriptionDefault
conc_bhConcentration of Conjugate Acid [BH⁺](M)0.01
conc_ohConcentration of OH⁻(M)0.01
conc_bConcentration of Base [B](M)0.1

How It Works

How to Calculate Kb

Formula

Kb = [BH⁺][OH⁻] / [B]

Where:

  • Kb = base dissociation constant
  • [BH⁺] = concentration of conjugate acid at equilibrium
  • [OH⁻] = hydroxide ion concentration at equilibrium
  • [B] = concentration of undissociated base at equilibrium

    • Larger Kb indicates a stronger base.

    Worked Example

    A 0.1 M ammonia solution has [NH4⁺] = 0.01 M and [OH⁻] = 0.01 M at equilibrium.

    conc_bh = 0.01conc_oh = 0.01conc_b = 0.1
    1. 01Kb = [BH⁺][OH⁻] / [B]
    2. 02Kb = (0.01)(0.01) / 0.1
    3. 03Kb = 0.0001 / 0.1
    4. 04Kb = 1.0 × 10⁻³

    Frequently Asked Questions

    What does Kb represent?

    Kb is the base dissociation constant. It measures the extent to which a base dissociates in water to produce hydroxide ions.

    How are Ka and Kb related?

    For a conjugate acid-base pair, Ka × Kb = Kw = 1.0 × 10⁻¹⁴ at 25°C.

    What is a typical Kb for a weak base?

    Weak bases have Kb values between about 10⁻² and 10⁻¹³. Ammonia has Kb = 1.8 × 10⁻⁵.

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